Sweden's 17th century royal warship Vasa, which sank in 1628 and was brought to the surface three centuries later, on Thursday began undergoing a major refurbishment, museum officials said.

The ship - which is housed in a special museum built over an old dry dock that is one of Stockholm's most popular tourist attractions - will receive an anti-corrosion treatment and get the 5,000 bolts lodged in its hull replaced.

"We need to remove the iron and the rust from inside the wood and replace (the bolts) by stainless steel that will not leak into the wood," Vasa Museum head Marika Hedin said.

The Vasa's hull was weakened by the pollution it was exposed to during the 333 years it spent on the Baltic Sea seabed after sinking in the Stockholm harbour on its maiden voyage.

The pollution, combined with the iron of the original bolts and rust, provoke "a chemical reaction that destroys the wood," Hedin explained.

The 5,000 new bolts, made of an alloy of chrome and nickel, will take five years to replace.

Neither the museum nor Swedish engineering firm Sandvik, which is providing the bolts, would say how much the refurbishment was set to cost.

Magnus Olofsson, who is in charge of the ship's conservation, explained the Vasa was built with many layers of oak, and that some of the bolts had to be two meters long, "to keep each piece of wood in the right place."

The Vasa was commissioned by Swedish King Gustav II Adolf in 1625 and was designed to be the mightiest warship in the world.

It was armed with 64 guns on two gun decks, but did not have enough ballast to counteract the weight and sank just minutes after setting sail on its maiden voyage.

A Swedish shipwreck enthusiast, Anders Franzen, located it in 1956 at a depth of 32 meters (105 feet).

The ship was raised in 1961, and the museum built around the ship opened in 1990. It is one of the most popular tourist sites in Scandinavia, with 1.1 million visitors annually.